Method, system and device for non-volatile memory device operation with low power, high speed and high density

ABSTRACT

Disclosed are methods, systems and devices for operation of non-volatile memory devices. In one aspect, a read operation or a particular write operation may be performed on a correlated electron switch (CES) device by coupling a terminal of the CES device to a particular node through any one of multiple different resistive paths.

BACKGROUND 1. Field

Disclosed are techniques for utilizing memory devices.

2. Information

Non-volatile memories are a class of memory in which the memory cell orelement does not lose its state after power supplied to the device isremoved. The earliest computer memories, made with rings of ferrite thatcould be magnetized in two directions, were non-volatile, for example.As semiconductor technology evolved into higher levels ofminiaturization, the ferrite devices were abandoned for the morecommonly known volatile memories, such as DRAMs (Dynamic Random AccessMemories) and SRAMs (Static-RAMs).

One type of non-volatile memory, electrically erasable programmableread-only memory (EEPROM) devices have large cell areas and may requirea large voltage (e.g., from 12.0 to 21.0 volts) on a transistor gate towrite or erase. Also, an erase or write time is typically of the orderof tens of microseconds. One limiting factor with EEPROMs is the limitednumber of erase/write cycles to no more than slightly over 600,000—or ofthe order of 10⁵-10⁶. The semiconductor industry has eliminated a needof a pass-gate switch transistor between EEPROMs and non-volatiletransistors by sectorizing a memory array in such a way that “pages”(e.g., sub-arrays) may be erased at a time in EEPROMs called flashmemory devices. In flash memory devices, an ability to keep randomaccess (erase/write single bits) was sacrificed for speed and higher bitdensity.

More recently, FeRAMs (Ferroelectric RAMs) have provided low power,relatively high write/read speed, and endurance for read/write cyclesexceeding 10 billion times. Similarly, magnetic memories (MRAMs) haveprovided high write/read speed and endurance, but with a high costpremium and higher power consumption. Neither of these technologiesreaches the density of flash memory devices, for example. As such, flashremains a non-volatile memory of choice. Nevertheless, it is generallyrecognized that flash memory technology may not scale easily below 65nanometers (nm); thus, new non-volatile memory devices capable of beingscaled to smaller sizes are actively being sought.

Technologies considered for the replacement of flash memory devices haveincluded memories based on certain materials that exhibit a resistancechange associated with a change of phase of the material (determined, atleast in part, by a long range ordering of atoms in the crystallinestructure). In one type of variable resistance memory called a phasechange memory (PCM/PCRAM) devices, a change in resistance occurs as thememory element is melted briefly and then cooled to either a conductivecrystalline state or a non-conductive amorphous state. Typical materialsvary and may include GeSbTe, where Sb and Te can be exchanged with otherelements of the same or similar properties on the Periodic Table.However, these resistance-based memories have not proved to becommercially useful because their transition between the conductive andthe insulating state depends on a physical structure phenomenon (e.g.,melting at up to 600 degrees C.) and returning to a solid state thatcannot be sufficiently controlled for a useful memory in manyapplications.

Another variable resistance memory category includes materials thatrespond to an initial high “forming” voltage and current to activate avariable resistance function. These materials may include, for example,Pr_(x)Ca_(y)Mn_(z)O_(ϵ), with x, y, z and ϵ of varying stoichiometry;transition metal oxides, such as CuO, CoO, VO_(x), NiO, TiO₂, Ta₂O₅; andsome perovskites, such as Cr; SrTiO₃. Several of these memory typesexist and fall into the resistive RAMs (ReRAMs) or conductive bridgeRAMS (CBRAM) classification, to distinguish them from the chalcogenidetype memories. It is postulated that resistance switching in these RAMsis due, at least in part, to the formation of narrow conducting paths orfilaments connecting the top and bottom conductive terminals by theelectroforming process, though the presence of such conducting filamentsare still a matter of controversy. Since operation of a ReRAM/CBRAM maybe strongly temperature dependent, a resistive switching mechanism in aReRAM/CBRAM may also be highly temperature dependent. Additionally,these systems may operate stochastically as the formation and movementof the filament is stochastic. Other types of ReRAM/CBRAM may alsoexhibit unstable qualities. Further, resistance switching inReRAM/CBRAMs tends to fatigue over many memory cycles. That is, after amemory state is changed many times, a difference in resistance between aconducting state and an insulative state may change significantly. In acommercial memory device, such a change may take the memory out ofspecification and make it unusable.

Given an inherent difficulty in forming a thin film resistance switchingmaterial that is stable over time and temperature, a workable resistanceswitching memory remains a challenge. Furthermore, all resistanceswitching mechanisms developed up to now have been inherently unsuitablefor memories, due to high currents, electroforming, no measurable memoryread or write windows over a reasonable range of temperatures andvoltages, and many other problems such as stochastic behavior. Thus,there remains a need in the art for a non-volatile memory that isdeterministic has low power, high speed, high density and stability, andin particular, such a memory that is scalable to feature sizes wellbelow 65 nanometers (nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Claimed subject matter is particularly pointed out and distinctlyclaimed in the concluding portion of the specification. However, both asto organization and/or method of operation, together with objects,features, and/or advantages thereof, it may be best understood byreference to the following detailed description if read with theaccompanying drawings in which:

FIG. 1A shows a plot of current density versus voltage for a CES deviceaccording to an embodiment;

FIG. 1B is a schematic diagram of an equivalent circuit to a CES deviceaccording to an embodiment;

FIG. 2 is schematic diagram of a circuit for applying operations to anon-volatile memory element according to an embodiment;

FIG. 3 is a timing diagram for operations applied to a non-volatilememory device according to a particular embodiment;

FIG. 4 is schematic diagram of a circuit for applying operations to anon-volatile memory element according to an alternative embodiment;

FIG. 5 is a timing diagram for operations applied to a non-volatilememory device according to an alternative embodiment;

FIG. 6 is schematic diagram of a circuit for applying operations tomultiple non-volatile memory elements according to an alternativeembodiment;

FIG. 7 is a timing diagram for operations applied to multiplenon-volatile memory devices according to an alternative embodiment; and

FIGS. 8 and 9 are schematic diagrams of circuits for applying operationsto multiple non-volatile memory elements according to an alternativeembodiment.

Reference is made in the following detailed description to accompanyingdrawings, which form a part hereof, wherein like numerals may designatelike parts throughout that are corresponding and/or analogous. It willbe appreciated that the figures have not necessarily been drawn toscale, such as for simplicity and/or clarity of illustration. Forexample, dimensions of some aspects may be exaggerated relative toothers. Further, it is to be understood that other embodiments may beutilized. Furthermore, structural and/or other changes may be madewithout departing from claimed subject matter. References throughoutthis specification to “claimed subject matter” refer to subject matterintended to be covered by one or more claims, or any portion thereof,and are not necessarily intended to refer to a complete claim set, to aparticular combination of claim sets (e.g., method claims, apparatusclaims, etc.), or to a particular claim. It should also be noted thatdirections and/or references, for example, such as up, down, top,bottom, and so on, may be used to facilitate discussion of drawings andare not intended to restrict application of claimed subject matter.Therefore, the following detailed description is not to be taken tolimit claimed subject matter and/or equivalents.

DETAILED DESCRIPTION

Particular aspects of the present disclosure incorporate a CorrelatedElectron Material (CEM) to form a correlated electron switch (CES). Inthis context, a CES may exhibit an abrupt conductor/insulator transitionarising from electron correlations rather than solid state structuralphase changes (e.g., crystalline/amorphous in phase change memory (PCM)devices or filamentary formation and conduction in resistive RAM devicesas discussed above). In one aspect, an abrupt conductor/insulatortransition in a CES may be responsive to a quantum mechanicalphenomenon, in contrast to melting/solidification or filament formation.Such a quantum mechanical transition between conductive and insulativestates in a CEM memory device may be understood in any one of severalaspects.

In one aspect, a quantum mechanical transition of a CES between aninsulative state and a conductive state may be understood in terms of aMott transition. In a Mott transition, a material may switch from aninsulative state to conductive state if a Mott transition conditionoccurs. The criteria may be defined by the condition(n_(C))^(1/3)a=0.26, where n_(C) is a concentration of electrons and “a”is a Bohr radius. If a critical carrier concentration is achieved suchthat the Mott criteria is met, a Mott transition may occur and state maychange from a high resistance/capacitance to a lowresistance/capacitance.

In one aspect, a Mott transition may be controlled by a localization ofelectrons. As carriers are localized, a strong coulomb interactionbetween electrons splits the bands of the material creating aninsulator. If electrons are no longer localized, a weak coulombinteraction may dominate band splitting, leaving behind a metal(conductive) band. This is sometimes explained as a “crowded elevator”phenomenon. While an elevator has only a few people in it, the peoplecan move around easily, which is analogous to a conducting state. Whilethe elevator reaches a certain concentration of people, on the otherhand, passengers can no longer move, which is analogous to theinsulative state. However, it should be understood that this classicalexplanation provided for illustrative purposes, like all classicalexplanations of quantum phenomenon, is only an incomplete analogy, andthat claimed subject matter is not limited in this respect.

In particular implementations of aspects of this disclosure, a resistiveswitching integrated circuit memory may comprise: a resistive switchingmemory cell including a CES device; a write circuit for placing theresistive switching memory cell in a first resistive state or a secondresistive state depending on signals provided to the memory cell,wherein a resistance of the CES is higher in the second resistance statethan in the first resistance state; and a read circuit for sensing thestate of the memory cell and providing an electrical signalcorresponding to the sensed state of the memory cell. In one aspect, aresistance of a CES in the second memory cell state may be more than 100times the resistance in the first memory cell state. In a particularimplementation, a CES device may switch resistive states responsive to aMott-transition in a majority of the volume of the CES device. In oneaspect, a CES device may comprise a material selected from a groupcomprising aluminum, cadmium, chromium, cobalt, copper, gold, iron,manganese, mercury, molybdenum, nickel, palladium, rhenium, ruthenium,silver, tin, titanium, vanadium, and zinc (which may be linked to acation such as oxygen or other types of ligands), or combinationsthereof.

In a particular embodiment, a CES device may be formed as a “CEM randomaccess memory (CeRAM)” device. In this context, a CeRAM device comprisesa material that may transition between or among a plurality ofpredetermined detectable memory states based, at least in part, on atransition of at least a portion of the material between a conductivestate and an insulative state utilizing the quantum mechanical Motttransition. In this context, a “memory state” means a detectable stateof a memory device that is indicative of a value, symbol, parameter orcondition, just to provide a few examples. In one particularimplementation, as described below, a memory state of a memory devicemay be detected based, at least in part, on a signal detected onterminals of the memory device in a “read operation.” In anotherparticular implementation, as described below, a memory device may beplaced in a particular memory state to represent or store a particularvalue, symbol or parameter by application of one or more signals acrossterminals of the memory device in a “write operation.”

In a particular implementation, a CES element may comprise materialsandwiched between conductive terminals. By applying a specific voltageand current between the terminals, the material may transition betweenthe aforementioned conductive and insulative memory states. A“programming signal” as referred to herein means a condition appliedacross terminals of a device to affect a physical state such as in awrite operation. In one example implementation, a programming signal maycomprise a current between terminals of a device and a voltage appliedacross terminals of the device to place the device in a particularimpedance state. As discussed in the particular example implementationsbelow, material of a CES element sandwiched between conductive terminalsmay be placed in an insulative or high impedance memory state byapplication of a first programming signal across the terminals having avoltage V_(reset) and current I_(reset), or placed in a conductive orlow impedance memory state by application of a second programming signalacross the terminals having a voltage V_(set) and current I_(set). Inthis context, it should be understood that terms such as “conductive orlow impedance” memory state and “insulative or high impedance” memorystate are relative terms and not specific to any particular quantity orvalue for impedance or conductance. For example, while a memory deviceis in a first memory state referred to as an insulative or highimpedance memory state the memory device in one aspect is lessconductive (or more insulative) than while the memory device in a secondmemory state referred to as a conductive or low impedance memory state.

In a particular implementation, CeRAM memory cells may comprise ametal/CEM/metal (M/CEM/M) stack formed on a semiconductor. Such anM/CEM/M stack may be formed on a diode, for example. In an example,implementation, such a diode may be selected from the group consistingof a junction diode and a Schottky diode. In this context, it should beunderstood that “metal” means a conductor, that is, any material thatacts like a metal, including, for example, polysilicon or a dopedsemiconductor.

FIG. 1A shows a plot of current density versus voltage across terminals(not shown) for a CES device according to an embodiment. Based, at leastin part, on a voltage applied to terminals of the CES device (e.g., in awrite operation), the CES may be placed in a conductive state or aninsulative state. For example application of a voltage V_(set) andcurrent density J_(set) may place the CES device in a low impedance orconductive memory state and application of a voltage V_(reset) and acurrent density J_(reset) may place the CES device in a high impedanceor insulative memory state. Following placement of the CES in aninsulative state or conductive memory state, the particular state of theCES device may be detected by application of a voltage V_(read) (e.g.,in a read operation) and detection of a current or current density atterminals of the CeRAM device.

According to an embodiment, the CES device of FIG. 1A may include anyTMO, such as, for example, perovskites, Mott insulators, charge exchangeinsulators, and Anderson disorder insulators. In particularimplementations, a CES device may be formed from switching materialssuch as nickel oxide, cobalt oxide, iron oxide, yttrium oxide, andperovskites such as Cr doped strontium titanate, lanthanum titanate, andthe manganate family including praesydium calcium manganate, andpraesydium lanthanum manganite, just to provide a few examples. Inparticular, oxides incorporating elements with incomplete d and forbital shells may exhibit sufficient resistive switching properties foruse in a CES device. In an embodiment, a CES device may be preparedwithout electroforming. Other implementations may employ othertransition metal compounds without deviating from claimed subjectmatter. For example, {M(chxn)₂Br}Br₂ where M may comprise Pt, Pd, or Ni,and chxn comprises 1R,2R-cyclohexanediamine, and other such metalcomplexes may be used without deviating from claimed subject matter.

In one aspect, the CES device of FIG. 1A may comprise materials that areTMO metal oxide variable resistance materials, though it should beunderstood that these are exemplary only, and are not intended to limitclaimed subject matter. Particular implementations may employ othervariable impedance materials as well. Nickel oxide, NiO, is disclosed asone particular TMO. NiO materials discussed herein may be doped withextrinsic ligands, which may stabilize variable resistance properties.In particular, NiO variable resistance materials disclosed herein mayinclude a carbon containing ligand, which may be indicated byNiO(C_(x)). Here, one skilled in the art may determine a value of x forany specific carbon containing ligand and any specific combination ofcarbon containing ligand with NiO simply by balancing valences. Inanother particular example, NiO doped with extrinsic ligands may beexpressed as NiO(L_(x)), where L_(x) is a ligand element or compound andx indicates a number of units of the ligand for one unit of NiO. Oneskilled in the art may determine a value of x for any specific ligandand any specific combination of ligand with NiO or any other transitionmetal simply by balancing valences.

If sufficient bias is applied (e.g., exceeding a band-splittingpotential) and the aforementioned Mott condition is met (injectedelectron holes=the electrons in a switching region), the CES device mayrapidly switch from a conductive state to an insulator state via theMott transition. This may occur at point 108 of the plot in FIG. 1A. Atthis point, electrons are no longer screened and become localized. Thiscorrelation may result in a strong electron-electron interactionpotential which splits the bands to form an insulator. While the CESdevice is still in the insulative state, current may generated bytransportation of electron holes. If enough bias is applied acrossterminals of the CES, electrons may be injected into ametal-insulator-metal (MIM) diode over the potential barrier of the MIMdevice. If enough electrons have been injected and enough potential isapplied across terminals to place the CES device in a set state, anincrease in electrons may screen electrons and remove a localization ofelectrons, which may collapse the band-splitting potential forming ametal.

According to an embodiment, current in a CES device may be controlled byan externally applied “compliance” condition determined based, at leastin part, on the external current limited during a write operation awrite operation to place the CES device in a conductive state. Thisexternally applied compliance current may also set a condition of acurrent density for a subsequent reset operation to place the CES in aninsulative state. As shown in the particular implementation of FIG. 1A,a current density J_(comp) applied during a write operation at point 116to place the CES device in conductive or low impedance state maydetermine a compliance condition for placing the CES device in aninsulative or high impedance state in a subsequent write operation. Asshown, the CES device may be subsequently placed in an insulative orhigh impedance state by application of a current densityJ_(reset)≥J_(comp) at a voltage V_(reset) at point 108, where J_(comp)is externally applied.

The compliance condition therefore may determine a number of electronsin a CES device which are to be “captured” by holes for the Motttransition. In other words, a current applied in a write operation toplace a CES device in a conductive memory state may determine a numberof holes to be injected to the CES device for subsequently transitioningthe CES device to an insulative memory state.

As pointed out above, a reset condition may occur in response to a Motttransition at point 108. As pointed out above, such a Mott transitionmay occur at condition in a CES device in which a concentration ofelectrons n equals a concentration of electron holes p. This conditionmay be modeled according to expression (1) as follows:

$\begin{matrix}{{{\lambda_{TF}n^{\frac{1}{3}}} = {C\text{\textasciitilde}0.26}}{n = ( \frac{C}{\lambda_{TF}} )^{3}}} & (1)\end{matrix}$where:

λ_(TF) is a Thomas Fermi screening length; and

C is a constant.

According to an embodiment, a current or current density in a region 104of the plot shown in FIG. 1A may exist in response to injection of holesfrom a voltage signal applied across terminals of a CES device. Here,injection of holes may meet a Mott transition criterion for theconductive state to insulative state transition at current I_(MI) as acritical voltage V_(MI) is applied across terminals of CES device. Thismay be modeled according to expression (3) as follows:

$\begin{matrix}{{{I_{MI}( V_{MI} )} = {\frac{{dQ}( V_{MI} )}{dt} \approx \frac{Q( V_{MI} )}{t}}}{{Q( V_{MI} )} = {{qn}( V_{MI} )}}} & (3)\end{matrix}$

-   -   Where Q(V_(MI)) is the charged injected (hole or electron) and        is a function of an applied voltage.

Injection of electron holes to enable a Mott transition may occurbetween bands and in response to critical voltage V_(MI). and criticalcurrent I_(MI). By equating electron concentration n with a chargeconcentration to bring about a Mott transition by holes injected byI_(MI) in expression (3) according to expression (1), a dependency ofsuch a critical voltage V_(MI) on Thomas Fermi screening length λ_(TF)may be modeled according to expression (4) as follows:

$\begin{matrix}{{{I_{MI}( V_{MI} )} = {\frac{Q( V_{MI} )}{t} = {\frac{{qn}( V_{MI} )}{t} = {\frac{q}{t}( \frac{C}{\lambda_{TF}} )^{3}}}}}{{J_{reset}( V_{MI} )} = {{J_{MI}( V_{MI} )} = {\frac{I_{MI}( V_{MI} )}{A_{CeRam}} = {\frac{q}{A_{CeRam}t}( \frac{C}{\lambda_{TF}( V_{MI} )} )^{3}}}}}} & (4)\end{matrix}$

Where:

-   -   A_(CeRam) is a cross-sectional area of a CES element; and    -   J_(reset)(V_(MI)) is a current density through the CES element        to be applied to the CES element at a critical voltage V_(MI) to        place the CES element in an insulative state.

According to an embodiment, a CES element may be placed in a conductivememory state (e.g., by transitioning from an insulative memory state) byinjection of a sufficient number of electrons to satisfy a Motttransition criteria.

In transitioning a CES to a conductive memory state, as enough electronshave been injected and the potential across terminal of the CES deviceovercomes a critical switching potential (e.g., V_(set)), injectedelectrons begin to screen and unlocalize double-occupied electrons toreverse a disproportion reaction and closing the bandgap. A currentdensity J_(set)(V_(MI)) for transitioning the CES to the conductivememory state at a critical voltage V_(MI) enabling transition to theconductive memory state may be expressed according to expression (6) asfollows:

$\begin{matrix}{{{I_{IM}( V_{IM} )} = {\frac{{dQ}( V_{IM} )}{dt} \approx \frac{Q( V_{IM} )}{t}}}{{Q( V_{IM} )} = {{qn}( V_{IM} )}}{{I_{IM}( V_{IM} )} = {\frac{Q( V_{IM} )}{t} = {\frac{{qn}( V_{IM} )}{t} = {\frac{q}{t}( \frac{C}{a_{B}} )^{3}}}}}} & (6)\end{matrix}$where:

AB is a Bohr radius.

According to an embodiment, a “read window” 102 for detecting a memorystate of a CES device in a read operation may be set out as a differencebetween a portion 106 the plot of FIG. 1A while the CES device is in aninsulative state and a portion 104 of the plot FIG. 1A while the CESdevice is in a conductive state at a read voltage V_(read). In aparticular implementation, read window 102 may be used to determine aThomas Fermi screening length λ_(TF) of material making up the CESdevice. For example, at a voltage V_(reset), current densities J_(reset)and J_(set) may be related to according to expression (7) as follows:

$\begin{matrix}{{\lambda_{TF}( {@V_{reset}} )} = {a_{B}( \frac{J_{reset}}{J_{off}} )}^{\frac{1}{3}}} & (7)\end{matrix}$

In another embodiment, a “write window” 110 for placing a CES device inan insulative or conductive memory state in a write operation may be setout as a difference between V_(reset) (at J_(reset)) and V_(set) (atJ_(set)). Establishing |V_(set)|>|V_(reset)| enables a switch betweenconductive and insulative state. V_(reset) may be approximately at aband splitting potential arising from correlation and V_(set) may beapproximately twice the band splitting potential. In particularimplementations, a size of write window 110 may be determined, at leastin part, by materials and doping of the CES device.

The transition from high resistance/capacitance to lowresistance/capacitance in a CES device may be represented by a singularimpedance of the CES device. FIG. 1B depicts a schematic diagram of anequivalent circuit of an example variable impeder device (such as a CESdevice), such as variable impeder device 124. As mentioned, variableimpeder device 124 may comprise characteristics of both variableresistance and variable capacitance. For example, an equivalent circuitfor a variable impeder device may, in an embodiment, comprise a variableresistor, such as variable resistor 126 in parallel with a variablecapacitor, such as variable capacitor 128. Of course, although avariable resistor 126 and variable capacitor 128 are depicted in FIG. 1Bas comprising discrete components, a variable impeder device, such asvariable impeder device 124, may comprise a substantially homogenousCEM, wherein the CEM comprises characteristics of variable capacitanceand variable resistance. Table 1 below depicts an example truth tablefor an example variable impeder device, such as variable impeder device100.

TABLE 1 Resistance Capacitance Impedance R_(high)(V_(applied))C_(high)(V_(applied)) Z_(high)(V_(applied)) R_(low)(V_(applied))C_(low)(V_(applied))~0 Z_(low)(V_(applied))

FIG. 2 is schematic diagram of a circuit for applying operations to anon-volatile memory element according to an embodiment. Here, thecircuit of FIG. 2 comprises a non-volatile memory element CeN, which maycomprise a CES element such as a CeRAM element as discussed above. Afirst terminal of Non-volatile memory element CeN is coupled to avoltage supply VDD and a second terminal of non-volatile memory elementCeN may be coupled to a common source voltage VSS through any one ofmultiple different resistive paths for particular write or readoperations. In this context, a “resistive path” means one or more paths,either alone or in combination, to permit current to flow between nodesin a network. Such a resistive path may comprise, for example,conductive material such as metals, semiconductive materials orstructures, resistive materials, just to provide a few examples. In oneexample, a resistive path may comprise conducting elements that may bein a particular open or closed state in combination with low resistancematerials such as metals. In addition, a resistive path may comprisemultiple paths through which current may flow between nodes. It shouldbe understood, however, that these are merely examples of a resistivepath, and claimed subject matter is not limited in this respect.

In one example, in a set operation to place non-volatile memory elementCeN in a low impedance or conductive state, a voltage of signal SETN maybe raised to close FET NS, providing a conducting element to couple thesecond terminal of non-volatile memory element CeN to common sourcevoltage VSS in a first resistive path. In this context, a “voltagesupply” means a circuit element that generates a voltage level at one ormore nodes relative to one or more reference nodes. In oneimplementation, a voltage supply may comprise a circuit to generate apredetermined voltage level. It should be understood, however, that thisis merely an example of a voltage supply, and claimed subject matter isnot limited in this respect. In this context, a “common source voltage”means one or more nodes maintained at a particular voltage level. In oneparticular implementation, a common source voltage may comprise avoltage maintained at one or more reference nodes such as a ground nodeor a node at 0.0V relative to a voltage maintained by a voltage supply.It should be understood, however, that this is merely an example of acommon source voltage, and claimed subject matter is not limited in thisrespect.

In this context, a “conducting element” comprises a circuit elementcapable of permitting current to pass between two nodes. In a particularimplementation, a conducting element may vary a current permitted topass between nodes based, at least in part, on a particular condition.The particular implementations described herein employ FETs asconducting elements to permit current to pass between source and drainterminals based, at least in part, on a voltage applied to a gateterminal. It should be understood, however, that these are merelyexamples of conducting elements in the description and drawings providedfor illustration, and that other types of devices such as, a bipolartransistor, diode, variable resistor, etc. may be used as a conductingelement, and that claimed subject matter is not limited this respect. Inthis context, a conducting element having first and second terminals may“connect” the first and second terminals by providing a conductive pathbetween the first and second terminals having a very small or negligibleimpedance for a particular signal. In one particular exampleimplementation, a conductive element may vary an impedance between thefirst and second terminals based, at least in part, on a signal providedto a third terminal of the conductive element (e.g., a based on avoltage or current applied to the third terminal). In one aspect, aconductive element may “close” to thereby connect first and secondterminals in response to a signal provided on the third terminal.Likewise, a conductive element may “open” to thereby disconnect firstand second terminals in response to a different signal provide on thethird terminal. In one aspect, a conductive element in an open state mayisolate a first portion of a circuit from a second portion of thecircuit by removing or disrupting a conductive path between the firstand second portions of the circuit. In another aspect, a conductingelement may vary an impedance between first and second terminals betweenopened and closed state based on a signal provided to a third terminal.

Diode coupled FETs NRD1 and NRD2 in series are closed and provide aresistive path in parallel with FET NS. Having a common drain-gateconnection, FETs NRD1 and NRD2 may each have a voltage drop equal to anFET turn-on voltage (e.g., each may impart voltage drop of a 0.4V).Accordingly, while FETs NS and NR are both in an open state and a dropacross each of FETs NRD1 and NRD2 is 0.4V, a voltage across non-volatilememory device CeN may be VDD−(VSS+2.0×0.4V).

A resulting voltage drop in a resistive path between the second terminalof non-volatile memory element CeN and common source voltage through FETNS may provide a voltage V_(set) sufficient to place non-volatile memoryelement CeN in a conductive or low impedance state in a set operation asdiscussed above. States of signals to place non-volatile memory elementCeN in a conductive or low impedance state may be shown in the timingdiagram of FIG. 3 between times 2 and 3 in which the voltage of signalSETN is raised while a voltage on signal RSTN remains low.

In a reset operation to place non-volatile memory element CeN in a highimpedance or insulative state, a voltage of signal SETN may bemaintained low (to place FET NS in an open state) while a voltage ofsignal RSTN may be raised to close FET NR. Closing FET NR may provide aconductive element to couple the second terminal of non-volatile memoryelement CeN to common source voltage VSS in a resistive path includingdiode coupled FET NRD2, in series with NR in parallel with diode coupledFET NRD1. Here, a resulting voltage drop between the second terminal ofnon-volatile memory element CeN and common source voltage, including avoltage drop across diode coupled FET NRD2 (e.g., approximately 0.4V)and negligible drop across closed FET NR, may provide a voltageV_(reset) and current I_(reset) sufficient to place non-volatile memoryelement CeN in an insulative or high impedance state in a resetoperation as discussed above. States of signals to place non-volatilememory element CeN in an insulative or high impedance state may be shownin the timing diagram of FIG. 3 between times 5 and 6 in which thevoltage of signal RSTN is raised while a voltage on signal SETN remainslow.

According to an embodiment, a read operation to detect a current stateof non-volatile memory element CeN may provide a particular signal VO atan output terminal indicative of the current state. Here, voltages ofsignals RSTN and SETN may be maintained low (placing FETs NS and NR inopen states leaving a single resistive path between the second terminaland VSS through diode coupled FETs NRD1 and NRD2 in series) while avoltage of signal VO may be sampled at the output terminal. This isillustrated in the timing diagram of FIG. 3 between times 4 and 5 toread a conductive or low impedance state, and between times 7 and 8 toread an insulative or high impedance state.

The particular implementation of FIG. 2 employs NFETs to provideconducting elements to form different resistive paths between the secondterminal of non-volatile memory element CeN and common source voltageVSS. FIG. 4 is schematic diagram of a circuit for applying operations toa non-volatile memory element according to an alternative embodiment inwhich PFETs are used to form resistive paths between a first terminal ofa non-volatile memory element CeP and voltage supply VDD. Here, commonsource voltage VSS is coupled to a first terminal of non-volatile memoryelement CeP while a second terminal of non-volatile memory element CePmay be coupled to voltage supply VDD by one or more resistive pathsformed by PFETs PS, PR, PRD1 and PRD2.

For example, in a set operation to place non-volatile memory element CePin a low impedance or conductive state, a voltage of signal SETP_B maybe lowered to close FET PS, providing a conductive element to couple thesecond terminal of non-volatile memory element CeP to voltage supply VDDin a first resistive path. Diode coupled FETs PRD1 and PRD2 have acommon drain-gate connection, each having a voltage drop equal to an FETturn-on voltage (e.g., each may impart voltage drop of a 0.4V). Being inseries, FETs PRD1 and PRD2 provide a resistive path in parallel with FETPS. Here, a resulting voltage difference between the second terminal ofnon-volatile memory element CeN and common source voltage may provide avoltage V_(set) and current I_(set) sufficient to place non-volatilememory element CeP in a conductive or low impedance state in a setoperation as discussed above. States of signals to place non-volatilememory element CeP in a conductive or low impedance state may be shownin the timing diagram of FIG. 5 between times 2 and 3 in which thevoltage of signal SETP_B is lowered while a voltage on signal RSTP_Bremains high.

In a reset operation to place non-volatile memory element CeP in a highimpedance or insulative state, a voltage of signal SETP_B may bemaintained high (to place FET PS in an open state) while a voltage ofsignal RSTP_B may be lowered to close FET PR. Closing FET PR may providea conductive element to couple the second terminal of non-volatilememory element CeP to voltage supply VDD in a resistive path includingdiode coupled FET PRD2 imparting a voltage drop (e.g., a voltage drop ofapproximately 0.4V), in series with diode coupled FET PRD1 in parallelwith FET PR. A voltage drop through diode coupled FET PRD1 in parallelwith FET PR may be negligible. A resulting voltage drop between thesecond terminal of non-volatile memory element CeP and voltage supplyVDD (primarily a voltage drop through diode coupled FET PRD2) mayprovide a voltage V_(reset) and current I_(reset) across terminals ofnon-volatile memory element CeP sufficient to place non-volatile memoryelement CeP in an insulative or high impedance state in a resetoperation as discussed above. States of signals to place non-volatilememory element CeP in an insulative or high impedance state may be shownin the timing diagram of FIG. 5 between times 5 and 6 in which thevoltage of signal RSTP_B is lowered while a voltage on signal SETP_Bremains high.

According to an embodiment, a read operation to detect a current stateof non-volatile memory element CeP may provide a particular signal VO atan output terminal indicative of the current state of CeP. Here,voltages of signals RSTP_B and SETP_B may be maintained high (placingFETs PS and PR in open states leaving a single resistive path betweenthe second terminal and VSS through diode coupled FETs PRD1 and PRD2 inseries) while a voltage of signal VO may be sampled at the outputterminal. This is illustrated in the timing diagram of FIG. 5 betweentimes 4 and 5 to read a conductive or low impedance state, and betweentimes 7 and 8 to read an insulative or high impedance state.

FIG. 6 is schematic diagram of a circuit for applying operations tomultiple non-volatile memory elements combining features of the circuitsshown in FIGS. 2 and 4 according to an alternative embodiment. Here,non-volatile memory elements CeN and CeP may be configured to be incomplementary states to represent different expressions, values,symbols, conditions or parameters, etc. (e.g., a “1” or “0”). Forexample, two complementary states of non-volatile memory elements CeNand CeP may represent either of two corresponding different expressions,values or symbols. For example, in a first complementary staterepresenting a first expression, value or symbol non-volatile memoryelement CeN may be maintained a low impedance or conductive state andnon-volatile memory element CeP may be maintained in a high impedance orinsulative state. In a second complementary state representing a secondexpression, value, symbol, condition or parameter may be represented bymaintaining non-volatile memory element CeN in a high impedance orinsulative state and non-volatile memory element CeP in a low impedanceor conductive state.

As may be observed, output signal VO_(N), having a voltage determinedbased on a current state of non-volatile memory element CeN, is providedto a gate of FET PO while output signal VO_(P), having a voltagedetermined based on a current state of non-volatile memory element CeP,is provided to a gate of FET NO. In the first complementary statementioned above, CeN is maintained a low impedance or conductive stateand non-volatile memory element CeP is maintained in a high impedance orinsulative state. This may occur in response to the write operationillustrated from time 2 to 3 in the timing diagram of FIG. 7. This mayplace voltages of output signals VO_(N) and VO_(P) on the gates of FETsPO and NO are high in a subsequent read operation at time 4 to 5 in FIG.7, opening FET PO and closing FET NO. In this state, a voltage of dataoutput signal DO is fixed at the common source voltage VSS. In thesecond complementary state mentioned above, CeN is maintained a highimpedance or insulative state and non-volatile memory element CeP ismaintained in a low impedance or conductive state. This may occur inresponse to the write operation illustrated from time 5 to 6 in thetiming diagram of FIG. 7. This may place voltages on the gates of FETsPO and NO low, closing FET PO and opening FET NO. In this state, avoltage of data output signal DO will be fixed at the voltage supply VDDin a subsequent read operation at time 7 to 8 in FIG. 7.

As may be observed, a voltage of data output signal DO is fixed to ateither common source voltage VSS or voltage supply VDD in readoperations, depending on whether non-volatile memory elements CeN andCeP are in the aforementioned first or second complementary state. Thismay avoid or eliminate use of complicated sense amplifier circuitry tosense impedance states of non-volatile memory elements CeN and CeP asrepresenting a particular expression, value, symbol, parameter orcondition.

As shown in the alternative implementation of FIG. 8, the circuit ofFIG. 6 may be simplified to eliminate terminals to receive signal RSTP_B(for a reset operation to place non-volatile memory element CeP in ahigh impedance or insulative state) and signal SETP_B (for a resetoperation to place non-volatile memory element CeP in a low impedance orinsulative state) by including inverters IS and IR responsive to signalsSET and RST.

In another alternative implementation as shown in FIG. 9, the circuit ofFIG. 6 may be simplified by combining output signals VO_(N) and VO_(P)to a single output signal VO′ applied to the gates of FETs NO and PO,and eliminating two diode coupled FETs. In a write operation to placenon-volatile memory elements CeN and CeP in the first complementarystate, FET NS may be closed and FET NR may be opened to allow a setoperation on non-volatile memory element CeN, and FET PS may be openedand FET PR may be closed to enable a reset operation on non-volatilememory element CeP. In a subsequent read operation, the voltage ofoutput signal VO′ may be high to fix the voltage of data out signal DOto common voltage source VSS. Similarly, in a write operation to placenon-volatile memory elements CeN and CeP in the second complementarystate, FET NS may be opened and FET NR may be closed to allow a resetoperation on non-volatile memory element CeN, and FET PS may be closedand FET PR may be opened to enable a set operation on non-volatilememory element CeP. In a subsequent read operation, the voltage ofoutput signal VO′ may be low to fix the voltage of data out signal DO tovoltage supply VDD.

Being in complementary impedance states, at any one time eithernon-volatile memory element CeN or non-volatile memory element CeP maybe in a conductive or low impedance state. During a read operation, toprevent an unintended reset operation on the non-volatile memory elementin the conductive or low impedance state, current through thisnon-volatile memory element may be limited to below a current levelI_(reset). In a read operation, FETs NS, PS, NR and PR may be in an openstate such that the current through non-volatile memory elements CeN andCeP is roughly VDD−VSS divided by the combined resistance ofnon-volatile memory elements CeN and CeP. Here, the particularnon-volatile memory element in the high impedance or insulative statemay significantly limit current through non-volatile memory elements CeNand CeP in a read operation so as to prevent an unintended resetoperation on the non-volatile memory element in the conductive or lowimpedance state.

References throughout this specification to one implementation, animplementation, one embodiment, an embodiment and/or the like means thata particular feature, structure, and/or characteristic described inconnection with a particular implementation and/or embodiment isincluded in at least one implementation and/or embodiment of claimedsubject matter. Thus, appearances of such phrases, for example, invarious places throughout this specification are not necessarilyintended to refer to the same implementation or to any one particularimplementation described. Furthermore, it is to be understood thatparticular features, structures, and/or characteristics described arecapable of being combined in various ways in one or more implementationsand, therefore, are within intended claim scope, for example. Ingeneral, of course, these and other issues vary with context. Therefore,particular context of description and/or usage provides helpful guidanceregarding inferences to be drawn.

While there has been illustrated and described what are presentlyconsidered to be example features, it will be understood by thoseskilled in the art that various other modifications may be made, andequivalents may be substituted, without departing from claimed subjectmatter. Additionally, many modifications may be made to adapt aparticular situation to the teachings of claimed subject matter withoutdeparting from the central concept described herein. Therefore, it isintended that claimed subject matter not be limited to the particularexamples disclosed, but that such claimed subject matter may alsoinclude all aspects falling within the scope of the appended claims, andequivalents thereof.

What is claimed is:
 1. A device comprising: a first non-volatile memoryelement comprising a first terminal coupled to a first node and a secondterminal; and one or more first conductive elements to selectivelycouple the second terminal of the first non-volatile element to a secondnode through a first selected resistive path so as to apply at least afirst voltage between the first and second terminals in a readoperation, and selectively couple the second terminal of the firstnon-volatile memory element to the second node through one or moresecond selected resistive paths so as to apply at least a second voltagebetween the first and second terminals in a write operation, wherein atleast a first of the one or more second selected resistive paths toapply a first programming signal between the first and second terminalsof the first non-volatile memory element to place the first non-volatilememory element in a first impedance state, and a least a second of theone or more second selected resistive paths to apply a secondprogramming signal between the first and second terminals of the firstnon-volatile memory element to place the first non-volatile memoryelement in a second impedance state, and wherein the first of the one ormore second selected resistive paths to enable sufficient currentdensity in the first non-volatile memory element on application of thefirst programming signal to equal or exceed a current density in thefirst non-volatile memory element on application of the secondprogramming signal.
 2. The device of claim 1, wherein: the first of theone or more second selected resistive paths to comprise the firstselected resistive path and a second conductive element in a closedstate and coupled in parallel with at least a portion of the firstselected resistive path to enable the sufficient current density in thefirst non-volatile memory element to place the first non-volatile memoryelement in the first impedance state, and wherein the second conductiveelement to be in an open state during the read operation; and at leastone of the one or more first resistive paths to comprise a series ofdiode coupled field effect transistors (FETs), and wherein the secondconductive element to comprise a FET coupled in parallel with at leastone of the diode coupled FETs.
 3. The device of claim 1, wherein thefirst programming signal to comprise a first programming signal voltageand a first programming signal current, and wherein the secondprogramming signal to comprise a second programming signal voltage and asecond programming signal current, wherein a magnitude of the firstprogramming signal current to be greater than a magnitude of the secondprogramming signal current, and wherein a magnitude of the secondprogramming signal voltage to be greater than a magnitude of the firstprogramming signal voltage.
 4. The device of claim 1, and furthercomprising: a second non-volatile memory element comprising a firstterminal coupled to the second node and a second terminal; and one ormore third conductive elements to selectively couple the second terminalof the second non-volatile memory element to the first node through athird selected resistive path so as to apply at least a third voltagebetween the first and second terminals of the second non-volatile memoryelement in the read operation, and selectively couple the secondterminal of the second non-volatile memory element to the first nodethrough one or more fourth selected resistive paths so as to apply atleast a fourth voltage between the first and second terminals of thesecond non-volatile memory element in the write operation, wherein: atleast a first of the one or more fourth selected resistive paths toapply a third programming signal between the first and second terminalsof the second non-volatile memory element to place the secondnon-volatile memory element in the first impedance state, and a least asecond of the one or more fourth selected resistive paths to apply afourth programming signal between the first and second terminals of thesecond non-volatile memory element to place the second non-volatilememory element in the second impedance state; and the first of the oneor more fourth selected resistive paths to enable sufficient currentdensity in the second non-volatile memory element on application of thethird programming signal to equal or exceed a current density in thesecond non-volatile memory element on application of the fourthprogramming signal.
 5. The device of claim 4, and further comprising: anoutput terminal; and one or more fourth conducting elements to couplethe output terminal to the first node or the second node based, at leastin part, on an impedance state of the first non-volatile memory elementor an impedance state of the second non-volatile memory element, or acombination thereof.
 6. The device of claim 4, wherein impedance statesof the first and second non-volatile memory elements responsive to thewrite operation to represent a first symbol, value, expression,condition and/or parameter if the first non-volatile memory element isin a high impedance or insulative state and the second non-volatilememory element is in a low impedance and/or conductive state, andrepresent a second symbol, value, expression, condition and/or parameterif first non-volatile memory element is in the low impedance and/orconductive state and the second non-volatile memory element is in thehigh impedance and/or insulative state.
 7. The device of claim 1,wherein the first non-volatile memory element to comprise a correlatedelectron switch (CES) element.
 8. The device of claim 1, wherein thefirst non-volatile memory element to comprise a correlated electronrandom access memory element.
 9. A method comprising: while a firstterminal of a first non-volatile memory element is coupled to a firstnode, selectively coupling a second terminal of the first non-volatilememory element to a second node through one or more first selectedresistive paths so as to apply at least a first voltage between thefirst and second terminals in a read operation; while the first terminalof the first non-volatile memory element is coupled to the first node,selectively coupling the second terminal of the first non-volatilememory element to the second node through a second selected resistivepath so as to apply at least a second voltage between the first andsecond terminals in a first write operation to place the firstnon-volatile memory element in a first impedance state; and while thefirst terminal of the first non-volatile memory element is coupled tothe first node, selectively coupling the second terminal of the firstnon-volatile memory element to the second node through one or more thirdselected resistive paths so as to apply at least a third voltage betweenthe first and second terminals in a second write operation to place thefirst non-volatile memory element in a second impedance state, whereinthe second of the one or more second selected resistive paths to enablesufficient current density in the first non-volatile memory elementduring the second write operation to equal or exceed a current densityin the first non-volatile memory element during the first writeoperation.
 10. The method of claim 9, wherein selectively coupling thesecond terminal of the first non-volatile memory element to the secondnode through the one or more second selected resistive paths so as toapply at least the second voltage between the first and second terminalsin the first write operation further to comprise applying a firstprogramming signal between the first and second terminals of the firstnon-volatile memory element to place the first non-volatile memoryelement in the first impedance state.
 11. The method of claim 9, whereinselectively coupling the second terminal of the first non-volatilememory element to the second node through the one or more third selectedresistive paths so as to apply at least the third voltage between thefirst and second terminals in the second write operation further tocomprise applying a second programming signal across the first andsecond terminals of the first non-volatile memory element to place thefirst non-volatile memory element in the second impedance state.
 12. Themethod of claim 11, wherein the first programming signal to comprise afirst programming signal voltage and a first programming signal current,and wherein the second programming signal to comprise a secondprogramming signal voltage and a second programming signal current,wherein a magnitude of the first programming signal current to be lessthan a magnitude of the second programming signal current, and wherein amagnitude of the second programming signal voltage to be less than amagnitude of the first programming signal voltage.
 13. The method ofclaim 9, and further comprising: while a first terminal of a secondnon-volatile memory element is coupled to the second node, selectivelycoupling a second terminal of the second non-volatile memory element tothe first node through one or more fourth selected resistive paths so asto apply at least a fourth voltage between the first and secondterminals in the read operation; and selectively coupling the secondterminal of the second non-volatile memory element to the first nodethrough one or more fifth selected resistive paths so as to apply atleast a fifth voltage between the first and second terminals of thesecond non-volatile memory element in the second write operation. 14.The method of claim 13, and further comprising coupling an outputterminal to the first node or the second node based, at least in part,on an impedance state of the first non-volatile memory element or animpedance state of the second non-volatile memory element, or acombination thereof.
 15. The device of claim 1, wherein the firstimpedance state to comprise a low impedance or conductive state and thesecond impedance state to comprise a high impedance or insulative state.16. The method of claim 9, wherein the first non-volatile memory elementto comprise a correlated electron switch (CES) element.
 17. The methodof claim 9, wherein the first non-volatile memory element to comprise acorrelated electron random access memory element.